1. Field of the Invention
The present invention relates to an image processing device and an image processing method which perform resolution conversion to a halftone image.
2. Description of the Related Art
There is known an image processing device which receives multi-value image data from an information processing device such as a host computer, executes halftone processing to the data and thereafter, transmits the data to an output device such as a printer for printing. When the resolution of the multi-value image is different from that of the output device, it is required to perform resolution conversion in the process of the image processing, but when the resolution of the multi-value image is equal to that of the output device, it is not required to perform the resolution conversion. Therefore, for example, in a case of outputting data produced using an application program on a host computer by a printer or the like, it is preferable for a printer driver in the host computer to generate the data fitted to the resolution of the printer.
However, in the following case, the resolution conversion is performed in an image processing device.
In a case of printing data with an output device of high resolution, when the image processing device requests an information processing device for the data fitted to the resolution of the output device, since an amount of the data which the image processing device receives from the information processing device is large, a processing speed of the image processing device is reduced. In this case, the image processing device generally requests the information processing device for data fitted to resolution lower than the resolution of the output device and processes the received data for expansion to be fitted to the resolution of the output device. In addition, in reverse, there is a case where the image processing device requests the information processing device for data fitted to resolution higher than the resolution of the output device and processes the received data for contraction to be fitted to the resolution of the output device.
Hereinafter, the latter case will be explained as an example.
In a typical output device, in most cases, the resolution in a main scanning direction is equal to that in a sub-scanning direction. However, in a case of a printing device using an electronic photograph system, when a rotational speed of a data printing portion (transfer drum or the like) is increased for increasing the printing speed of the printing device, the resolution in the sub-scanning direction is degraded. For example, upon increasing the rotational speed of the data printing portion by 1.5 times in the output device having the resolution of 600 dpi in each of the main scanning direction and the sub-scanning direction, the resolution in the sub-scanning direction becomes 400 dpi which is two-thirds of 600 dpi. In this case, it is assumed that the image processing device requests the information processing device for data fitted to the resolution of main scanning resolution of 600 dpi×sub-scanning resolution of 400 dpi so as to be fitted to the resolution of the output device. However, since an application program in the information processing device is possibly not adapted for the resolution which differs between a longitudinal direction and a lateral direction, the image processing device can not necessarily obtain the correct data. Therefore, the image processing device requests the information processing device for data having the resolution of 600×600 dpi in each of the longitudinal direction and the lateral direction and converts the resolution of the received data into the resolution of the output device. That is, the image processing device converts the resolution of main scanning resolution of 600 dpi×sub-scanning resolution of 600 dpi into the resolution of main scanning resolution of 600 dpi×sub-scanning resolution of 400 dpi.
In the aforementioned resolution conversion, there are two methods of a method performing resolution conversion to a multi-value image and a method of performing resolution conversion to a binary image (image obtained by performing halftone processing to a multi-value image) (for example, refer to Japanese Patent Laid-Open No. H01-136465(1989) or Japanese Patent Laid-Open No. S63-102553(1988)). Further, the method of performing the resolution conversion to the binary image includes a method of simply performing expansion and contraction to the binary image according to a given rule. Further, there is a method where a binary image is converted into a multi-value image in accordance with resolution of the output device, halftone processing is executed to the multi-value image to again generate a binary image, and resolution conversion is performed to the binary image.
Since the halftone processing is executed to the multi-value image after the resolution conversion in the method of performing the resolution conversion to the multi-value image, a tone quality of the binary image is not affected by the resolution conversion. However, this method has a problem that a storage region for resolution conversion processing is increased in size because of performing the resolution conversion to the multi-value image. On the other hand, in a case of the method of performing the resolution conversion to the binary image, the storage region for resolution conversion processing maybe made small in size. However, upon performing resolution conversion such as simple expansion processing or simple contraction processing to the binary image, there occurs the problem that moiré is generated in a halftone concentration region of the multi-value or an intermediate concentration of the multi-value is made to be changed without being held. In addition, in a case of a method where the binary image is converted into the multi-value image in accordance with the resolution of the output device, the halftone processing is executed to the multi-value image to again generate the binary image, there occurs the problem that the processing is complicated and the processing time is increased because of performing the halftone processing twice.